1. Field of the Invention
The present invention relates to a balanced transmission termination device comprising a common mode choke coil for restraining common mode noise in a balanced transmission system in which a paired cable is employed, and to a receiver unit composed of such a device.
2. Description of the Related Art
Referring to FIG. 4, a balanced transmission system with a conventionally balanced transmission termination device includes a transmitter unit 30 and a receiver unit 31. Transmitter unit 30 and receiver unit 31 carry out communications through balanced transmission line 32 consisting of a pair of leads 36 and 37.
Receiver unit 31 contains a conventionally balanced transmission termination device 42 and a main body circuit 33. Balanced transmission termination device 42, connected at the termination of balanced transmission line 32, is made up of a common mode choke coil 34 and an impedance element R.
Common mode choke coil 34 attenuates common mode noise that has been mixed into the desired signal on balanced transmission line 32. Details of this operation are described hereunder.
Referring now to FIG. 5(a), when a current Id flowing through the pair of leads 36 and 37 is of an opposite phase (differential mode), the respective magnetic fluxes 38 and 39 generated by the current are equal and opposite, and thus are mutually erased. Therefore, common mode choke coil 34 does not influence the transmission signals.
Referring now to FIG. 5(b), when a current Ic flowing through the pair of leads 36 and 37 is in-phase (common mode), magnetic fluxes 40 and 41 add together, instead of cancelling. This increases the apparent impedance of common mode choke coil 34 to common mode currents, without affecting the out-of-phase differential currents.
As a result of the foregoing, common mode choke coil 34 attenuates only the common mode current (common mode noise) Ic that has been mixed as noise, without affecting differential mode current (differential signal) Id, which is the object to be transmitted.
Japanese Unexamined Patent No.Hei-7-240652 also discloses a related prior art. This prior art is related to a balanced-to-unbalanced transducer comprising a transformer (corresponding to a common mode choke coil).
Common mode choke coil 34 utilizes the effect of the magnetic fluxes generated by a current flowing through the coil. Therefore, when an excessive amount of common mode current Ic flows through common mode choke coil 34, the core of common mode choke coil 34 becomes saturated. Saturation disturbs the effective performance of common mode choke coil 34. Also, an excessive amount of common mode current Ic may damage thin coils having a small diameter.
Especially when the aforementioned balanced transmission system is employed in a moving vehicle such as automobile or aircraft, the system is prone to cause the mentioned problems since the engine in an automobile or aircraft generates a large amount of common mode electrical noise. In addition, such a moving vehicle as stated above often passes in proximity to a radio transmitter tower or a high-tension cable. These sources are also capable of causing common mode problems.
In view of the foregoing, an object of the present invention is to provide a balanced transmission termination device that can restrain saturation of a common mode choke coil and prevent damage thereof and a receiver unit using such a balanced transmission termination device.
The first aspect of the invention provides a balanced transmission termination device at the termination of a balanced transmission line consisting of a pair of leads, comprising: two primary impedance elements connected in series between the pair of leads with a nodal point therebetween that is grounded; a common mode choke coil disposed posterior to the primary impedance elements for attenuating a common mode noise generated in the balanced transmission line; a secondary impedance element disposed posterior to the common mode choke coil for impedance matching with the primary impedance elements; wherein the primary impedance elements have an impedance that can maintain balanced transmission in a differential mode; the primary and secondary impedance elements have an impedance that allows impedance matching with a characteristic impedance of the balanced transmission line; and the impedance of the common mode choke coil, primary impedance elements and a secondary impedance element are determined in such a manner that a common mode current flowing into the common mode choke coil will not exceed a predetermined value.
By such a construction, excessive common mode current is prevented from flowing into the common mode choke coil. As a result, saturation and damage of the common mode choke coil is prevented even in the presence of an excessive common mode current in the balanced transmission line.
The second aspect of the invention provides a balanced transmission termination device according to the first aspect, wherein each of the primary impedance elements is a resistance element.
By such a construction, a balanced transmission termination device that can prevent the saturation and damage of the common mode choke coil is easily obtained.
The third aspect of the invention provides a receiver unit provided with a balanced transmission termination device according to the first aspect.
By such a construction, an excessive common mode current is prevented from flowing into the common mode choke coil, and saturation and damage of the common mode choke coil is prevented. As a result, the performance of the common mode choke coil is maintained, and influence of common mode noise is minimized even when an excessive common mode current is generated in the balanced transmission line.
The fourth aspect of the invention provides a receiver unit, in addition to the third aspect, wherein each of the primary impedance elements disposed in the balanced transmission termination device is a resistance element. By such a construction, a receiver unit that minimizes the influence of common mode noise is easily obtained.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.